contextify: tie lifetimes of context & sandbox

When the previous set of changes (bfff07b4) it was possible to have the
context get garbage collected while sandbox was still live. We need to
tie the lifetime of the context to the lifetime of the sandbox.

Fixes: https://github.com/nodejs/node/issues/5768
PR-URL: https://github.com/nodejs/node/pull/5786


Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>